Direct fired cylindrical bell type furnace



c. E. PEC'K May 26, 1942.

2,284,014 DIRECT FIRED CYLINDRICAL BELL TYPE FURNACE Filed Oct. 7, 1939 3 Sheets-Sheet 1 I I I I I l/ ,1

z I 1 I I I 1 1/ a 01/ 1/1 iI I/ INVENTOR C arawce A? Per/{1 WITNESSES 0 1 WZM ATTORNEY May 26, 1942.

c. E. PECK DIRECT FIRED CYLINDRICAL BELL TYPE FURNACE Filed Oct. 7* 3 sheets-sheet .2

WITNESSES INVEN'II'OR. (742197760 Z. Peck.

ATTORNEY 6. E. PECK May 26, 19421 DIRECT FIRED CYLINDRICAL BELL TYPE FURNACE File'd Oct. 7', 1939 3 SheetsSheet 3 INVENTOR 6707792762? A. Pea/ 1 WITNESSES: I

ATTORNEY Patented May 2 1942 j UNITED. 1 STATES PATENT OFFICE DIBE C'I: FIRED TYPE I V Clarence Peck, Wilkinsburg, Pa, assignor to ,Westinghouse Electric & Manufacturing Oom- Dany, East Pittsburgh,

Pennsylvania Pa.', a corporation of Application October 7, 1939, SerialNo. 298,456

7 Claims.

Bell furnaces are known and generally comprise a refractory-metal inner hood placed about spacing between rows upwardlyof the bell. The

- bell is provided with flues at the top thereof and a charge to be heat treated, the hood being encased by an outer insulating bellwith suflicient space between the hood and the interior of the bell to permit the disposition of heating means therein. My invention relates, generally, to bell furnaces which are to be direct-fired with com bustible fluid fuel, but which will lend themselves to accurate control and distribution of the temperature within their'heating chambers, the degree of accuracy of such'control and distribution being comparable to bell furnaces heated by electrical heating means usually in the form of sinuously-shaped resistor ribbons.

While direct-fired bell furnaces of the type described are known, they have heretofore been employed only where relatively wide. temperature discrepancies could be tolerated, since it has been found extremely difficult to secure accurate control and distribution of the temperature within a bell type furnaces employing direct gas-firing as heating means, Accordingly, it is an object of my invention to provide a bell furnace construction adapted to be heated directly by gas-firing in the space between the interior of .the bell and the hood, and which will have features enabling accurate temperature control and distribution within the furnace.

In the copending application of J. H. Germany, Serial No. 298,461, filed concurrently herewith, and assigned to the Westinghouse Electric 8: Manufacturing Company, there is disclosed adirect gas-fired bell fln'nace having features assuring a completely sealed combustion space between the hood and bell, whereby the combustion of the fuel is not affected by secondary or unwanted air that might seep into or be drawn into the com-' bustion space through'crevices at the bottom of the furnace, and disrupt the controlled proportions of burning gas and air admitted to the furname through the gas burners. In embodiments of my invention I employed the aforesaid invention described in the said application to advantage.

It is an object of my invention to provide a direct gas-fired bell furnace having a multiplicity since the products of combustionrise to the top and give off heat as they rise, the burners are preferably adjusted 'so that the greater propore tion of the heat units is supplied by the lower burners with correspondingly lesser units supplied by the upper burners. In this way, I avoid I over-heating of the top of the hood.

Another'object or embodiment of my invention is to provide a direct gas-fired bell furnace in which the burners are vertically distributed, and the bell and hood so constructed that the combustion space increases in size in the direction of the flow of the products of combustion so that as additional burning gases are fed to the combustion space, additional rooms is provided for them. Actually, this feature of my invention is accomplished ,by increasing the area of the combustion space in the direction transverse to the general flow of the productsof. combustion.

A still further object of my invention is to divide the vertically-disposed burners into horizontal control zones, each of which is separately controlled by a temperature responsive means so that the amount of heat units supplied by the burners in each horizontal zone can be controlled to the end that the desired temperature control and distribution in the furnac are secured.

Still further features of my invention lie in the construction of the outer metallic shellfor the bell, which enables the use of standard burners. This feature of my invention is achieved, in

accordance with one embodiment of my invention, by constructing the upright wall of the outer shell or casing of vertically-disposed curved sections of metal alternating with vertically-disposed angle members having. at least one flat side, these elements being securely welded together gastight, Standard burners are usually supplied with planar flanges, and in accordance with'my invention the flat side of an angle cooperates with the flange of a burner so that the two can be of gas burners distributed throughout the bell,

the burners being either individually, or ingroups,

readily controllable so that the heatingof the furnace throughout the extent .of the combustion space can be accurately and readily controlled.

secured together, the angle being so located that the burner will fire substantially tangential of the hood. I

Other features, objects-and innovations of invention will 'be apparent from the following description, taken in conjunction with the drawings which somewhat diagrammatically show furnaces embodying the improvements of my invention. In these drawings:

Figure 1 is a view in vertical cross-section sub-- stantially through the center of a bell furnace embodying features of my invention, and showshowing the manner in which the outer casing for the bell is constructed, and further showing.

the manner in which the burners are secured to the outer casing Fig. 5 is a sectional view of a modiflcation'of the detail of Fig. 4 showing a different form of casing structure, and a different manner of securing another type of burner thereto; and

Fig. 6 is a view in vertical cross-section of a bell type furnace showing a combustion space between the hood and bell of the furnace which increases in area upwardly of the furnace, the other details of the furnace being purposely omitted since they may follow generally those shown in the other figures. Fig. 6 also shows a different disposition of the flues for discharging the products of combustion.

In many respects, the bell furnace of my invention follows customary design in providing a foundation 2 upon-which rests the base of the furnace, the base being indicated in its entirety by the reference numeral 4. The base, in general, comprises a structural steel framework, represented by the reference numeral 8, supported by the foundation, and underlying a circular base plate 8 which is preferably heavy stock iron or steel, and consequently, gas-tight. A refractory and insulating hearth I0 is suitably supported on the base plate 8, and comprises a horizontal portion l2, completely covering the base plate, hav-' ing a central upstanding part I4 .in which heatresistant metal members 16 are imbedded for supporting a skeleton framework l8.on which the charge. to be heat-treated is disposed.

- The upstanding part l4 of the hearth and the skeleton framework l8 are generally cylindrical, and the gas-tight hood, indicated in its entirety by the reference numeral 28, is disposed on the base 4. The hood generally-comprises an upstanding cylindrical portion 22 having a lower end slightlyJarger than the upstanding part I4 of the hearth and the framework l8, and having a top 24 so that any charge resting on the chargesupporting framework I8 is completely encased by the hood and the base. In the illustrated embodiment of my invention, the lowermost portion of the hood is formed with an outwardly-directed, in this case, horizontal, annular skirt member 28 extending beyond the base and provided with a downwardly-extending flange 28 about its complete periphery. The flange protrudes into a trough 80 welded gas- 2,aa4,o14 I the refractory lining define the combustion chamber or gas space 84.

The construction of the furnace thus far described may be deemed to be somewhat conventional and in ordinary practice, the bell is provided with eye-bolts or some equivalent device 42 foundation 2, the lower part'of the bell being provided I with outwardly-directed brackets 48 having eyes 48 through which the pins may pass. Usually, the charge inside the hood is heated while enveloped by a protective atmosphere having'a composition which depends upon the heattreating process being carried out. For example, the atmosphere might be one which is entirely inert to the charge; or in the case of nitriding or the like, may have acomposition to' carry out certain desired reactions therewith. Such a protective atmosphere may be admitted under the hood through an inlet pipe 50 and suitably circulated by a fan 52. An outlet pipe 84 for the protective gas may be provided for the atmosphere within the hood so that it may be purged when a heat treatment is started and so that the atmosphere may be maintained clean tight to the base plate 8 of the base, and which also extends completely around the periphery of the base plate.

A bell, indicated in its entirety by the reference numeral 82, is adapted to be placed over the hood and generally follows the contour of the upper portion of the hood but is somewhat larger so as to provide a combustion chamber or gas space 34 between the two. The bell comprises an outer, gas-tight metal shell or casing 38 which completely encases it,.and flrebrick walls comprising a non-metallic inner refractory 38 backed by a heat-insulating brick 40. The inner surfaces of and pure during the heat-treating process. The fan 82 for circulating the atmosphere within the hood may be operated by any suitable mechanism such as, for example, a motor 58 driving a shaft 58 to which the fan is secured.

The pipes and 84 are gas-tightly sealed, as by welding, to the base plate at the points where they pass through it, and the shaft may be provided with an oil seal, such as shown and described in the application of Frederick R. J. Davis, Serial No.'221,719, flied July 28, 1938, now Patent No. 2,265,027, issued December 2, 1941, and assigned to the Westinghouse Electric 8: Manufacturing Company, although any other seal may be employed which would assure a gastight joint between the shaft 58 and the base plate 8.

A thermocouple pipe 88 also extends through the base plateand protrudes inside the hood for substantially its full length, and has, in this case, three vertically-spaced thermocouple Junctions so that, in effect, the charge-treating space inside the hood is divided into three horizontal vrtically displaced temperature control zones which will be later discussed. The pipe 80 is also secured to the base plate 8 where it passes v through it in a gas-tight manner. Accordingly, the base plate 8 isgas-tight throughout its extent, and by providing a seal between the base plate and hood flange, the charge-treating chamber inside the hood is made fully gas-tight.

For heating the furnace in accordance with my invention, a multiplicity of gas burners 88 are dispersed throughout the bell and, preferably, are arranged in vertically-spaced, horizontal, circumferentially-di'sposed rows, each of the burners shown in Fig. 1 being indicative of a circumferential row, andFig. 2 showing five burners comprising each horizontal row. The use of sevof the'circumferential rows of burners, but it is alsopossible to zone the rows so that, one or more of them is included ina control zone; and

for final adjustment each indiyidual burner in each circumferential row may, in turn; bev ad- ,Iustecl; ck I It may be observed that the lowermost circumferential row of burners is disposedslightly be; low the top of the charge supporting framework I8 so'that the bottom of the charge 'is thoroughly heated, and that each succeeding row above means 62, Gland 66. r l

The junctions 62, 64 and 6-6 are parts of tem- 1 and the last zone includes the row of burners 18 under control of the thermocouple junction 66.

Accordingly, the burners in the different zones are, controlled directly in accordincewith the temperature distribution in each of these zones, as determined .by

the thermocouple function I perature'responsive means for controlling the the lowermost row is spaced from therow below 1 a distance which increases, the greater the height of the row above the base of the furnace.

, Thus, consideringrow III as the lowermost, row

I2 is a predetermined distance above it while Jrow I4 is a. greater distance above .row 12 and row 16 a still greater distance above row 14, while the distance between the row 16 and "I8 is the greatest of all. In a practical embodiment of my invention, the lowermost row would be rated to yield approximatelyas much heat units as the two rows 12 and '14 immediately above it, with the upper row 14 yielding somewhat less heat'units than the row I2. The upper rows .16 and 18 would'supply the remaining heat units required ,which would be, in this case-,"a small fraction; approximately one-third, ;and even less, of the total heat required. Th'ereason for this adjustment lies in the fact that it is desired to heat the bottom of the charge completely. This requires a considerable amount of in the lower horizontal 'rows than are disposed in the upper horizontal rows. However, for'the purposes of uniformity and economy of construcflow of fuel to the different rows of burners in the respective control zones", the temperature 'responsive means for each zone further including a temperature control instrument 8 2 for the lowermost control zone, atemperature'control instrument 84, for the intermediate control zone,

and a temperature control instrument 86 for the top control zone. These control instruments may be of any well known type, and controlmotoroperated valves 38, 90 and 92 for the respective zones to control the amount offuel fed to the different rows of burners in each "of the control zones. Forexample; the instrument may be'of the type which repeatedly operates the supply valve to either on or "off-position, Or it may be of the type more commonly known as a proportioning control which allows the supply valve to move in small steps. If desired, additional temperature responsive means may have their tion, I prefer to have the same number of burn-.. ers in each horizontal row, and to adjust them;

andv control the amount of gas fed to them properly.

The fuel supplied to the difierent horizontal rows burns in the combustion space 34, and is temperature measuring .elements disposed directly in the combustion space for controlling the maximum temperature which may be attained therein, as is well known in the art.

As more completely explained in the. aforesaid application of J. H. Germany, to obtain very accurate control of the temperature in a directfired gas furnace of the bell type, it is preferable to seal the lower part of the combustion space, and especially where the bell rests on the'base, in order to preventseepage of air into the combustion .space through the crevices between the .base (or hood) and the bottom of the bell, the

seepage being accentuated, particularly in high furnaces, by the chimney-like effects due priultimately discharged as products of combustion through an enlarged central discharge flue- 80 in the embodiment shown in Fig. 1. shown in my embodiment are preferably'of a pre-mix type, that is, the combustible gas and air in proper proportions are first mixed and then supplied to the pipe system leading to each burner.

In the operation of my furnaces, I prefer to avoid direct impingement of the flame on the hood so that hot spots on the hood are avoided, which would cause rapid deterioration. To this end; each of the burners fires tangentially in the combustion space 34 so that the flames swerve toward the refractory lining 38. This feature, combined with the use of a multiplicity of burners, each of relatively low capacity but in the aggregate of a total capacity suflicient for the heat-treating process, increases the life of the furnace hood.

The burners The use of a multiplicity of burners in the marily to the great temperature difference between the'burning g'ases on the inside of the furnace and the air on the outside.

- In the furnace shown in Fig. 1, the bell is provided with a bottom frame'plate 94 which rests on a relatively narrow frame ring 96, which may be welded to the bottom of the frame plate 94, the weight of the bell being transmitted to the supporting structure through the frame ring 96, the hood skirt 26 and the'base. I Accordingly, the outer casing 36 of the bell is extended downwardly below the frame plate 94 to provide a downwardly-depending flange indicated in its entirety by the reference numeral 98 which extends into the trough 32. The trough contains a liquid sealing means I00 so that the combustion space 34 is completely sealed by virtue of the .two flanges 28 and 98 being immersed therein.

Since the combustion space sealed against the entry of air other than that admitted through the fuel supply, it is quite apparent that the combustion can be very accurately controlled. The supply of the fuel is determined substantially en-, tirely bythe temperature control instruments operating upon the supply valves in the fuel supply lines, assuming constant pressure and,proportioning conditions. By providing the numerous vertically-displaced temperature controlled zones, distribution of heat is also accurately determinable and controllable.

As a matter of economy, I prefer to employ standard burners. in my As shown in Figs. 2 and 4, one form of such burners comprises a burner block-J02 throughwhich 'a,,fuel any charge pipe IIIlextends terminating at a nozzle I05 in the burner block; Each burner maybe provided with a-valve III'I for individual control of the burners Secured. as by'weldingto the dischargepipe I III: atasuitable'distance from the discharge end of the pipe isaplanar flange I" of" the burner arranged substantially p findicular to the" pipe I. Inordertoaccommodatm such burners and provides gas-tight outer-cas-, ing for the bell, I prefer to construchthecasing 36 as a sectional shell comprising a, plurality of curved members IIIl alternating with ,a pinrality of angle'members 2, all of which are vertically disposed and extend the full height of the bell.

As shown more of the angle members are welded to the edgesof the curved members, the, weld being gas-tight along its full length so that the bottom of the angles and the curved members form the sealing flange 9B. A suitable top I is, of course, welded gas-tight to the upper edges of the curved members and angle members to complete, the casing. The angle members II2 are so disposed that one of their sides H6 is directed along a line. subparticularly in Fig. 2, the-edges f stantially perpendicular to a line tangent to an I imaginary cylinder substantially at'the middle' of the space between the hood 20-and the refractory lining 38 of the bell. a

The angle sides IIS are provided with a plu-;

rality of apertures, aligned with the different circumferential rows of burners through which bolts II8 extend, their heads being welded gastightly to the inner surfaces of anglesides 6.: A small reenforcing plate I20 maybe gas-tightly welded at the outer surfaceof the. angle sides".

IIS and also provided with holes-alignedwith'.

those in the angle sides 6, so'tha't. the shank; of the bolts. I I! extend therethrougli. 'TI'o secure the burners in position, theirplanar flanges III! are suitably drilled to fit over theishanks of the bolts I20 and are clamped to* the reinforcingplate by means of nuts I22, gasketmeans' I24 being preferably disposed between the reinforcing plates and flanges to assure a gas-tight joint. Consequently the casing 26 can be considered airtight exceptfor the flue or flues which open into the atmosphere around the bell.

Fig, 5 indicates a modified form for securing burners to the casing. In this embodiment, the outer casing isa complete cylinder having holes drilled at suitable points for the reception of burners I26 which have angularly-shaped discharge pipes-128 provided with planar flanges I30. .Because of the large radiusof the outer casing, the reenforcing plates I 32 may be suitably welded thereto at the points where the burners extend through the casing, although, if desired, for smaller furnaces, their inside surfaces may be machined or otherwise formed to fit the curvature of the outer metallic casing; With respect to the burner shown in Fig. 5, itjis, of course, obvious that the bend in the'discharge pipe must be at suchan angle that the end of the pipe inside the bell is properly directed for the tangential firing desired.

Fig. 6 indicates'a slight refinement of the furnace built in accordance with my invention, it

being understood that this furnace has burners bustion space I tapers graduallywith the parrowest areaat the bottom of the furnace and the the largest area at the top thereof for the purpose of providing additional room for the products of combustion as they are added progressively by the higher rows of theburners. As an additionalmodiflcation, the furnace of Fig. 6 may have a plurality oflsmaller flues I38 disposed so that the amount of v fuel to each row can be set manually, and thereafter controlled automat- T ically by the temperature responsive means including thethermocouples, the temperature control instruments and the motor operated valves. By using a multiplicity of burners of relatively low capacity individually, but which in their aggregate supply therequired amount of heat, I avoid overheating of the hood as an additional advantage of this construction. While I have described burners of a premix: type, it is obvious that diiferent'types of burners may also be employed as, for example, luminous flame burners in' which the, gas and air are separately intro-' duced into the'burner or the combustion chamber. 1

In general, the combustion space between the bell and'the hood is' made'large enough so that the combustion will properly take place in the volume of space allotted between each row of burners and preferably in a manner so that the .flame from the premix. type of burners will not directly impinge on the' hood.- For many purposes, acombustion spacebetween the hood and the walls may be constructed of uniform crosssectional .area as shown in FigQl, but for some purposes, and especially forextra high tempera- 1 him furnaces, it is preferable totaper this phase t'ofaliow for the additional products of combustion resulting from the supply of fuel from the upper rows of burners.

In the preferred operation of practically all furnaces, the lower rows of burners supply most of the heat so that the amount of heat required as the rows of burners progress upwardly is less. In some embodiments, itmay be preferred to omit the circulating means comprising the fan 52, in which case the zonedtemperature control type furnace adapted for accurate temperature accurate temperature control and distribution through the furnace.

While I, have shown my invention in theform which I- now believe to embody the best practical details thereof, it is obviousthat other forms and many equivalents are conceivable.

, I claim as my invention:

1. A vertically-disposed directly gas-fired bell control and distribution therein, comprising a base adapted to support a charge to be heattreated; a gas-tight metallic-refractory, inner hood adapted to be placed about said charge and cooperating with said base to enclose said charge; a removable, insulating, walledbell about said hood, cooperating with said base, and having an inner wall spaced from said hood to provide a closed combustion space with flue provisions in the top of said bell for discharging the products of combustion, said'hood and bell being so constructed and arranged that said combustion space to be placed about said chargeand cooperating.

. .upwardly of the bell; burner means in said bell for supplying combustion fluids, comprising 8.

increases in cross-sectional area with increased distance upwardy of the bell; burner means in said bell for supplying combustion fluids, comprising a combustible fluidand oxygen-containing gas, to said space. saidgburner means comprising a multiplicity of burners distributed vertically in the wallsof said bell, whereby the combustion space increases in size to accommodate "increased volume or products of combustion in the direction of said flue provisions.

2. A vertically-disposed directlygas-flred bell type furnace comprising a base adapted to supportra charge to be heat-treated: a gas-tight, metallic-refractor, cylindrical inner hood adapted arranged that said combustion space increases in cross-sectional area with increased distance combustible fluid and oxygen-containing gas, to

' said space, said burner means comprising a multiplicity' of burners distributed vertically in the walls of said bell, whereby the combustion space increases in size to accommodate increased volume-of products of combustion in the direction of said flue provisions, said burners flringgenerally tangentially of said hood.

3. A vertically-disposed fluid-fired bell-type furnace comprising a base adapted to support a charge to be heat-treated; a gas-tight heatresistant inner hood adapted to be placed about said charge, and cooperating with said base to enclose said charge; a removable, insulating, walled bell having aninner wall spaced from said hood to provide a gas-space, said furnace having flue provisions communicating with said gas-space for the discharge of gaseous products of combustion; heating means in said bell comprising a multiplicity of burners distributed in the walls of said bell, for discharging hot gases directly into said gas-space, certain of said burners being'distributed at different places spaced in the general direction of gas-flow to said flue provisions; said hood and said bell being so spaced and arranged that the said gas-space increases in cross-sectional area with increased distance toward said flue provisions, whereby an increased size of gas-space is available to accommodate increased volume of gaseous products of combustion in the direction of said flue provi- SlOIlS.

t. A vertically-disposed directly gas-fired belltype furnace comprising, charge-enclosing means for supporting and gas-tightly enveloping a charge, said charge-enclosing means comprising an insulating, charge-supporting base, and a gastight, metallic-refractory hood adapted to be placed about a charge on said base; an outer, insulating, removable, bell cooperating with said charge-enclosing means, said bell having an inner heat-resistant wall spaced from said charge-- enclosing means for providing a closed gas-space, said bell having flue provisions in the top thereof, communicating with said gas-space, for the discharge of gaseous products'of combustion, said bell having an outer metallic casing which is airtight except forsaid flue provisions; heating means in said bell for supplying heating gases v 3 directly to said gas-space, said heating-means comprising a multiplicity of burners grouped in a plurality of vertically spaced zones; control means comprising temperature-responsive means in the respective zones, for automatically controlling the supply of heating ases supplied to saidgas-space, by the burners of the corresponding zones; and liquid sealing. meansgastightly sealing the lower end of said gas-space,

0 said liquid sealing means comprising means 1'01- holding a body of liquid in closed loop form generally following the periphery of the 'base, and

means-operative for insertion into the said bodyof liquid, gas-tightly secured to the outer casing of said bell, and generally following the ,periphery of said'outer casing; the two last-recited means and said charge-enclosing means cooperating to prevent the entry of extraneous gases into said gas-space at the bottom thereof.

5. A'vertically-disposed directly gas-fired belltype furnace comprising, charge enclosing means for supporting and gas-tight enveloping a charge,

.said' charge-enclosing means comprising an in'- sulating, charge-supporting base, ands, gas-tight, metallic-refractory hood adapted to be placed about a charge on said base; an insulating, removable bell cooperating with said charge-enclosing means, said bell having an inner heat-resistant wall spaced from said charge-enclosing means to provide a closed gas-space, said bell having flue provisionsin the top thereof, communicating with said gas-space, for the discharge of gaseous products of combustion, said bell having an outer metallic casing which is air-tight exceptfor said flue provisions; heating means in-said bell for supplying heating gases directly to said gas-space,

said heating means comprising a multiplicity of burners grouped in a plurality of vertically spaced zones; control means comprising temperatureresponsive means in the respective zones, for automatically controlling the supply of heating gases supplied to said gas-space by the burners of the corresponding zones; and liquid sealing means gas-tightly sealing the lower end of said gasspace, said liquid sealing means comprising means for holding a body of liquid in closed. loop form generally following the periphery of the vase, and means operative for insertion into the said body of liquid, gas-tightly secured to the outer casing of said bell, and generally following the periphery of said outer casing; the two lastrecited means and said charge-enclosing means cooperating to prevent the entry of extraneous gases into said gas-space at the bottom thereof, said hood and said bell being so spaced and arranged that the said combustion space increases in cross-sectional area with increased distance upwardly of the bell, whereby an increased size of combustion space is available to accommodate increased volume of gaseous products of combustion in the direction of said flue provisions.

6. In a bell furnace of the type in which the bell is placed about and removed from a charge before and after a heat-treatment, respectively;

an outer generally-cylindrical bell comprising a fire brick inner wall and an outer generally cylindrical casing therefor, said casing comprising a plurality upwardly-extending casing sections horizontally alternating with upwardly-extending angular sections, said casing sections and said angular sections being substantially the flat sides adapted to have burner flange secured thereto, said flat'sides also being substantially thellength of the casing,

7. In a bell furnace of the type in which the bell is removable -irom about a heat-treated charge, an outer generally cylindrical bell comprising a flre brick inner wall and an outer generally cylindrical casing therefor, 'said casing comprising a plurality of upwardly-disposed curved sections horizontally alternating with upwardly-disposed angular sections, each at said 10 angular sections having a flat side;-said curvedfor the reception of burner tubes: a plurality .of

burners having burner tubes extending through said holes, and substantially-planar flanges about said tubes;.and meansfor securing said flanges to said sides. 

